Foraminiferan shells, or tests, are built of hollow chambers
separated by partitions, with small openings called foramina that
connect the chambers (they get their name from these foramina).
The final chamber (the last one added) has an opening or openings
to the exterior, called the aperture. The living organism fills
all the chambers in its shell except for one or two of the most
recently constructed chambers. Most species build shells with
multiple chambers (multilocular) but some species build shells
with only a single chamber (unilocular). Click
here
to learn more about the
most common types of chamber arrangements.

Each of the major groups of foraminifera uses different materials
to build their shells. The basic types of wall structures are:

agglutinated -- test made of particles cemented together.
Some species use whatever particles are available, while other
species may select only sponge spicules or mica flakes or a
certain size particle to build their test.